Peierls critical mass formula

I'm doing a project for my 3rd year maths degree about the development of the nuclear bomb and the mathematicians that contributed. I have so far wrote about early experiments that led to the discovery of radioactivity, the michelson-morley experiment and Einsteins Special relativity. I am writing a chapter about peierls but am having a lot of trouble finding any information about the formula he derived concerning critical mass. Could anyone give me link to any websites that contain info about peierls' critical mass formula or any books that will help